xe2x80x9cNot Applicablexe2x80x9d
1. Field of the Invention
This invention relates to an improved vapor phase process for the catalytic oxidation of propylene to acrolein using as oxidant reducible particulate solids in an oxidized state, and where the resulting reduced solids are separately regenerated using molecular oxygen.
2. Description of Related Art
An important route to acrolein is the vapor phase oxidation of propylene over a multicomponent catalyst containing molybdenum and/or other metals, usually as their oxides. The reaction step involves oxidation of propylene with air (oxygen) to form acrolein, along with carbon oxides, water and smaller amounts of other oxidized byproducts. Typically the reaction is carried out in multitubular fixed-bed reactors. The large exothermic heat of reaction and the thermal sensitivity of the propylene oxidation requires low feed concentrations, expensive heat transfer equipment, handling of a large volume of gas, and good reactor temperature control. Low propylene concentration is also required to avoid flammability conditions.
The magnitude of some of these problems is reduced when a fluidized-bed reactor is used. The temperature can be readily controlled within a few degrees because of the intensive solids mixing and the good heat transfer characteristics. Higher propylene concentrations can be used because the danger of flammability is reduced by introducing the propylene directly into the reactor rather than premixing it with air (oxygen). However, very high propylene concentrations and low oxygen-to-propylene ratios in the reactor may result in the over reduction of the solids and reduced selectivity to acrolein. Also, significant back-mixing of gases in the fluidized-bed reactor result in poorer contact between gases in the bubbles and the solids, making it difficult to obtain high propylene conversion.
Modified forms of fluidized-bed reactor are known as recirculating solids reactor, transport bed reactor, transport line reactor, riser reactor, fluidization reactor, multi-chamber fluidized bed reactor, and by other names, depending on design and/or personal preference. In this application we will use the term xe2x80x9ctransport bed reactorxe2x80x9d to mean any reactor in which solid particles are injected at one end of the reactor and carried along with gas reactants at high velocities and discharged at the other end of the reactor to a gas-solids separation vessel. A riser reactor, in which the reactor is a vertical pipe wherein the reactive solids and gases are fed in at the bottom, transported in essentially plug flow and removed at the top, is one example of a transport bed reactor. Another example is a pipeline reactor, in which the flow of solids and gases is other than vertically upwards. A transport bed reactor, as defined herein, includes a riser reactor or pipeline reactor which also incorporates a zone for fluidization; i.e., a zone where the gas velocities are sufficiently high to carry out a substantial portion of the solids fed, but with more back-mixing of solids than would occur in plug flow. We will use the term xe2x80x9crecirculating solids reactor systemxe2x80x9d to mean a general reaction system with two reaction zones, in which two separate reactions take place, and which uses a particulate solid which circulates between the two reaction zones and takes part in both reactions. Optionally, either or both reaction zones may take place in a transport bed reactor or a fluidized bed. Such reaction systems have found use in catalytic cracking in petroleum refining and in other reactions.
U.S. Pat. No. 4,102,914 discloses a process for the preparation of acrylonitrile by passing a mixture comprising gaseous oxygen, propylene and ammonia, together with an ammoxidation catalyst, in a transport bed reactor while controlling the superficial linear gas velocity and solids feed rate at specific rates.
European Patent Office Publication No. 0 034 442 discloses a process for preparing unsaturated aldehydes by passing an unsaturated olefin and an excess of gaseous oxygen into a transport bed reactor with a solid oxidation catalyst at a linear gas velocity of 1.5 to 7.5 meters/second to achieve substantially plug flow within the reactor. Reaction products are stripped from the catalyst with steam in the stripper chamber.
U.S. Pat. No. 4,668,802 discloses a process for preparing maleic anhydride by oxidizing butane using an oxidized vanadium-phosphorous oxide catalyst as oxidant rather than oxygen wherein the resulting reduced catalyst is separately regenerated, and the use of a recirculating solids reactor system for this reaction. Certain of the examples use a transport bed or riser reactor for the butane oxidation reaction. Japanese Kokai 3-170,445 discloses a similar process for preparing acrolein and acrylic acid by oxidizing propane using an oxidized bismuth-molybdenum catalyst as oxidant.
The concept of using propylene in a similar process to make acrolein was disclosed in a paper titled xe2x80x9cOxidation and Ammoxidation of Propylene over Bismuth Molybdate Catalystxe2x80x9d, J. L. Callahan et al, Ind. Eng. Chem. Prod. Res. Develop., Vol. 9, No. 2 (1970). The use of a bismuth molybdate composition as direct oxidant was tested, but under the conditions of their tests this process was judged unsatisfactory because of the large amount of solids requiring circulation. Instead a process of using the bismuth molybdate composition as oxidation catalyst (rather than as direct oxidant) was chosen for commercialization. This paper does not disclose the improved reaction conditions of the present invention.
U.S. Pat. Nos. 4,152,393 and 4,341,717 disclose a specific design of reactor which it is said could be used, among a variety of applications, for the oxidation of propylene to acrolein using an oxidized solids as oxidant and regenerating the resulting reduced solids in its regeneration zone. A process example shows the ammoxidation of propylene using ammonia and an oxidized molybdenum-based catalyst as oxidant. The reactor consists of a single shell containing a reaction zone and a regeneration zone, using a specific design containing a first up-leg, a first down-leg, a second up-leg, a second down-leg and a return leg such that fluidized solids may be transferred from one zone to the other by one route and back by a second route, and so that the gases from one zone are not transferred to the other zone. This reactor has a complicated design which offers numerous places for potential plugging and which limits the ability to independently monitor and control oxidation zone and reduction zone conditions. This patent does not disclose the improved reaction conditions of the present invention.
The concept of using an oxidized catalyst to oxidize propylene was also disclosed in a paper titled xe2x80x9cModeling of Propylene Oxidation in a Circulating Fluidized-bed Reactorxe2x80x9d, G. S. Patience et al., at a conference named xe2x80x9cNew Developments in Selective Oxidation IIxe2x80x9d, and published by Elsevier Science B.V. (1994). However, while the theoretical model of this system demonstrated that it had potential use as an alternate reactor system for propylene oxidation, it listed numerous challenges and uncertainties for development of a working process.
U.S. Pat. No. 4,604,370 discloses a process for regenerating a spent molybdenum-bismuth based multi-oxide catalyst resulting from its use for the oxidation of propylene to acrolein by heating it in air to 380 to 500xc2x0 C. for at least 12 hours or to 500 to 540xc2x0 C. for at least 2 hours.
An advertising folder prepared by E. I. DuPont in 1973 titled xe2x80x9cChemical Technologies Worldwidexe2x80x9d included a single sheet titled xe2x80x9cTransport Bed Reactor Technology for Selective Processesxe2x80x9d, which described the general advantages of a transport bed or riser reactor, listing among typical applications the reaction of propylene to make acrylic acid and the reaction of propylene and ammonia to make acrylonitrile.
None of the above references disclose the necessary information to enable the economical use of a vapor phase process for the oxidation of propylene to acrolein using as oxidant particulate solids in an oxidized state, and where the resulting reduced solids are separately regenerated using molecular oxygen.
The preparation of multicomponent compositions containing molybdenum and/or other metals and their use as catalysts in the oxidation of propylene to make acrolein is well known in the art. For example, U.S. Pat. Nos. 4,677,084 and 4,769,477 disclose a process for making highly attrition resistant silica-based catalysts containing molybdenum, vanadium or other metals. The molybdenum catalyst composition described was stated to show good catalytic performance in a conventional process for making acrylonitrile from propylene and ammonia. Numerous other patents such as U.S. Pat. No. 3,631,099, GB 1,490,489 or JP 05,301,051 also disclose specific catalyst compositions containing molybdenum for use in the oxidation of propylene to acrolein in a fixed-bed or fluidized-bed process.
The present invention relates to a process for the selective vapor oxidation of propylene to acrolein, comprising the steps of: (a) contacting a feed gas containing from 1 mol % to 100 mol % (preferably from 5 mol % to 30 mol %) propylene, 0 to 20 mol % oxygen, 0 to 70 mol % water, and the remainder inert gas with an effective amount of a bismuth molybdate multimetal oxide in oxidized form comprised of particles from 10 to 300 micrometers in size, in a transport bed reactor at a temperature of 250 to 450xc2x0 C., a gas residence time in the reaction zone from 1 second to 15 seconds, and a solids residence time in the reaction zone from 2 seconds to 60 seconds to convert at least a portion the propylen to acrolein using the bismuth molybdate multimetal oxide solids in oxidized form as a direst oxidant; (b) removing the effluent produced in the transport bed reactor of step (a) and separating the resultant reduced solids from the effluent gases (preferably stripping off any effluent gases from the reduced solids), transporting the reduced solids to a regenerator zone of the recirculating solids reactor system, and recovering acrolein from the effluent gases; (c) oxidizing the reduced bismuth molybdate multimetal oxide in the regenerator zone using an oxygen-containing gas, at a temperature of 250 to 500xc2x0 C. at a solids residence time in the regenerator zone of 0.5 minute to 10 minutes, and at an oxygen-containing gas residence time from 3 seconds to 30 seconds; and, (d) recycling the oxidized bismuth molybdate multimetal oxide produced in step (c) to the transport bed reactor.
It is an object of this invention to provide an improved vapor phase process using a transport bed reactor for the oxidation of propylene to acrolein using the oxidized form of an attrition resistant solid containing molybdenum, and where the resulting reduced solids are separately regenerated using gaseous oxygen.